Expansion and connection of Early Jurassic Ocean Anoxia: Constraints from U and Mo isotopes

Reconstructing ancient ocean redox conditions is essential for understanding the link between marine anoxia and environmental changes. The Early Jurassic experienced significant oceanic redox fluctuations during the Pliensbachian/Toarcian event (Pl/To; ~184 Ma) and the more pronounced Toarcian Oceanic Anoxic Event (T-OAE; ~183 Ma). However, the expansion and connectivity of anoxic waters, e.g., in the European epicontinental seaways (EES), remain poorly understood. Here, we aim to apply U and Mo isotopes on carbonates and black shales that have become a powerful geochemical proxy for tracking redox changes at both local and global scales, combined with other redox-sensitive trace elements and REY, to unravel the oceanic redox evolution of the Early Jurassic.

Carbonate sediments investigated here are from Peniche (Portugal) and Toumliline (Morocco), covering both the Pl/To and T-OAE intervals. Shale-normalized rare earth element and yttrium patterns in the Peniche samples are characterized by negative Ce, positive Gd, and positive Y anomalies, indicating precipitation from open marine seawater. Recent data from near-contemporaneous Italian carbonate deposits suggest more extensive global seafloor anoxia during the T-OAE [1]. Our preliminary data, however, do not show negative U isotope shifts in the Peniche section suggesting that there was no global expansion of seafloor anoxia during either the Pl/To event or the T-OAE. Complementary U data of the Morocco section and coupled Mo-U isotope data from three different basins of the EES may clarify whether the redox shifts occurred contemporaneously and whether they represent a local or global phenomenon.

[1] Remírez et al. (2024), PNAS, 121(27)